Quantum plasmons and intraband excitons in doped nanoparticles: Insights from quantum chemistry

We use excited-state quantum chemistry techniques to investigate the intraband absorption of doped semiconductor nanoparticles as a function of doping density, nanoparticle radius, and material properties. Modeling the excess electrons as interacting electrons confined to a sphere, we find that the excitation evolves from single-particle to plasmonic with increasing number of electrons at fixed density, and the threshold number of electrons to produce a plasmon increases with density due to quantum confinement and electron-hole attraction. In addition, the excitation passes through an intermediate regime where it is best characterized as an intraband exciton. We compare equation-of-motion coupled-cluster theory with those of more affordable single-excitation theories and identify the inclusion of electron-hole interactions as essential to describing the evolution of the excitation. Despite the simplicity of our model, the results are in reasonable agreement with the experimental spectra of doped ZnO nanoparticles at a doping density of 1.4 × 10²⁰ cm^-3. Based on our quantum chemistry calculations, we develop a schematic model that captures the dependence of the excitation energy on nanoparticle radius and electron density.